I'm fascinated by Acala,however,I couldn't find out much about him.I'm not sure whether he's an enlightened being or a wordly protector or what.

A person once asked me why I would want to stop rebirth. "It sounds pretty cool. Being able to come back. Who wouldn't want to be reborn."I replied. "Wanting to be reborn is like wanting to stay in a jail cell, when you have the chance to go free and experience the whole wide world. Does a convict, on being freed from his shabby, constricting, little cell, suddenly say "I really want to go back to jail and be put in a cell. It sounds pretty cool. Being able to come back. Who wouldn't want that?"

Red Faced Buddha wrote:I'm fascinated by Acala,however,I couldn't find out much about him.I'm not sure whether he's an enlightened being or a wordly protector or what.

Acala means Immovable in Sanskrit. He is depicted (in Tibetan Buddhism) rising with one knee raised and the other leg still on the floor. He has a raised (I think flaming) sword (and this is true in TB and Japanese Buddhism).

Tiendai seems to heavily emphasize Acala. In Tibetan Buddhism he is generally found in outer Tantra. The qualities cultivated are protection but also immovability in terms of spiritual attainment and environmental protection. In TB he seems to be enlightened rather than a worldly deity (also in TB he is not exactly a protector). There is also more than one form in TB.

The famous Rinzai monk Takuan Soho (d. 1645) describes Fudo in terms of the habit of the mind to "stop" or attach to objects, which itself constitutes delusion (mayoi). From Fudochi Shimmyo Roku ("Record of the Wondrous Mind of Immovable Wisdom"):

There is a Buddhist statue called Fudo Myo-o. He is represented holding a sword in his right hand and a rope in his left. He is baring his teeth and his eyes glare penetratingly. He stands ferociously to destroy the evil spirits or demons who try to harm or hinder the Buddha's teachings. Although he is seen assuming this form he is not hiding on this earth. He is the symbolic protector of Buddhism. His body embodies Immovable Wisdom and is made for all sentient beings to see.

The simple-minded, out of awe, will refrain from hindering the Buddhist teachings. The people who are approaching enlightenment realize that Fudo Myo-o symbolizes Immovable Wisdom as the destroyer of delusions. He who becomes enlightened and carries on his life as exemplified by Fudo Myo-o will not be touched even by evil spirits.

In other words, Fudo Myo-o is the unmoving "One Mind" of everyone. This also refers to the "Immovable Body". Not to move means not to stop with whatever object one encounters. Not to move means not to stop with an object that is seen. Because if the mind stops on any object it will be disturbed with thoughts and emotions. This will lead to movement in the heart and mind. The stopping inevitably leads to the movement which is disturbance; therefore there will be no freedom of movement.

Fudo Myoo, aka Ayra Acalanatha, is a wisdom deity said to be an emanation of Vairocana Buddha. He is HUGE in Japan, particularly in Shingon, but also in Tendai, I am told. Here's the information on him from the Shingon Buddhist Institute:

At the Shingon Temple I attend, the head priest once described him as embodying compassion despite his wrathful appearance, because he protects us from delusion and uses his rope to bind "demons" (delusions) and bring them to the dharma.